A VoIP (Voice over IP) speech codec is required to have good packet loss robustness. For example, with embedded variable bit-rate speech encoding (EV-VBR) being promoted by the ITU-T (International Telecommunication Union—Telecommunication Standardization Sector) as a next-generation VoIP codec, subjective quality of decoded speech required under frame loss conditions has been established based on subjective quality of error-free decoded speech.
Of decoded speech signal quality degradation due to frame loss, that which most affects sound reception quality is degradation related to power fluctuations involving loss of sound and excessively loud sound. Therefore, in order to improve frame loss compensation capability, it is important for a speech decoding apparatus to be able to decode suitable power information with a lost frame.
To enable a speech decoding apparatus to decode correct power information in the event of a frame loss, measures are taken to improve the ability to conceal lost power information by transmitting lost frame power information from a speech encoding apparatus to a speech decoding apparatus as redundant information. For example, with the technology disclosed in Patent Document 1, by transmitting decoded speech signal power as redundant information, the power of decoded speech generated by concealment processing is matched to decoded speech signal power received as redundant information. In order to perform matching to decoded speech signal power, excitation power is calculated back using received decoded speech signal power and impulse response power of a synthesis filter configured by means of a linear prediction coefficient obtained by concealment processing.
Thus, according to the technology disclosed in Patent Document 1, decoded speech signal power is used as redundant information for concealment processing, making it possible to match decoded speech signal power at the time of frame loss concealment processing to decoded speech signal power in an error-free state.    Patent Document 1: Japanese Patent Application Laid-Open No. 2005-534950